Various Department of Energy sites, such as the Y-12 Plant in Oak Ridge, Tenn., used mercury extensively for the production of weapons. Some of the mercury used at these sites has been released to local aquatic systems. It is estimated that approximately 330 metric tons of mercury was discharged from the Y-12 plant to the local environment, between 1953 and 1963. Further, some other sites, such as the Savannah River are contaminated with mercury from industrial plants like chloro-alkali plants, which produced chlorine. Coal-fired power plants generate various gaseous pollutants, such as sulfur dioxide, carbon dioxide, and heavy metals. The majority of these pollutants are responsible for adverse effects on humans, animals, and plants. The proposed Clean Air Interstate Rule (CAIR) requires the reduction of the emission of sulfur dioxide to the environment. Sulfur dioxide is one of the components responsible for acid rain. Since sulfur emission rules are becoming increasingly stringent, more and more power plants are installing wet flue gas desulfurization (FGD) systems for SO2 control. One of the co-benefits of these systems is that they capture mercury and other trace metals from the flue gas. However, most of these systems have a water blowdown stream, which leads to the discharge of mercury. Since mercury discharge limits are becoming more stringent, there is a need for highly efficient water treatment technologies to remove mercury prior to discharging the FGD waste stream to the surface water.
In aquatic systems, mercury is bio-accumulated up the aquatic food chain reaching toxic concentrations in large fish and the animals that feed on fish. In an EPA study, it was observed that the accumulation factor (water column to base tissue) in the Savannah River is 4,000,000, i.e. methylmercury is 4 million times more concentrated in the fish tissue than in the water column. The EPA has set the maximum contaminant level goals (MCLG) for mercury to be 2 ppb.
Over the years, a number of different sorbents have been developed to capture mercury from aquatic systems. These include activated charcoal, amine-containing polymers, ion-exchange resins, chelating resins, modified clays, zeolites, pozzolana, modified silica [Soliman, E. M., Saleh, M. B., Ahmed, S. A., Anal. Chem. Acta, v523, 2004, 133] and modified alumina. One of the major problems with some of these sorbents, such as ion-exchange resins, is the lack of selectivity which makes them less efficient in the presence of competing ions such as Ca2+ and Mg2+. Further, a high amount of total dissolved solids (TDS) makes ion exchange resins less effective. A major issue with the currently used sorbent materials is the lack of accessibility of the mercury ions to the internal surface area where the chelating/coordinating ligands are present, leading to reduced mercury sorption capacity despite a high specific surface area (m2/g).